We are studying the structure and transcriptional regulation of the genes that code for ribosomal RNA in the frog Xenopus. We have located the sites of transcription initiation and termination and have determined their nucleotide sequence. These sequences will now be selectively altered to determine precisely which nucleotides are involved in promoter and terminator function. Altered molecules will be assayed by injection into oocytes and spreading the injected molecules for electron microscopy. Work will also continue on developing an in vitro transcription system using homogenates of oocyte nuclei. A third project will be to study the transition from inactive to active chromatin structure after DNA is injected into oocytes.